Several techniques for anodizing aluminum have been developed in the prior art and have consisted generally of placing a positive charge on a continuous aluminum web that is then passed through an anodizing cell for electrolytically depositing an aluminum oxide coating on the surface of the aluminum web. In an early prior art technique, the positive charge was placed on the aluminum web by passing a web over a conductive contact roller, such as is shown in U.S. Pat. No. 3,563,863 and U.S. Pat. No. 2,538,317. A significant problem with the contact roll method has been arcing between the roll and the aluminum web which causes pitting of the aluminum and deterioration of the contact roll.
Another known technique for introducing a positive charge on the aluminum web includes passing the web through a contact cell containing anodes and an electrolyte which electrically connects the annodes to the web, such as is shown in U.S. Pat. No. 3,920,525. A major problem with the contact cell method is that "burning" occurs in the anodizing cell caused by the surge of the current into the web as it enters the anodizing cell. It will be seen that a need to prevent arcing in the contact roller technique and a need to prevent burning in the contact cell technique have limited the maximum current densities that can be applied in the anodization process and therefore have limited the production rates that can be obtained.
In prior art systems using the contact cell technique, the configuration of electrodes has typically been one wherein either all the anodes in the contact cell and all the cathodes in the anodizing cell have been on the same side of the web or both the anodes and the cathodes have been evenly distributed on both sides of the web in both cells. Also, it is generally accepted in the anodization art that the anodized coating on the web should be sealed with hot water at a temperature, for example, of 200.degree. F., as taught in U.S. Pat. No. 2,538,317.